One construction of a slant plate type compressor, particularly a wobble plate compressor, with a variale capacity or variable displacement mechanism which is suitable for use in an automotive air conditioner is disclosed in the U.S. Pat. No. 3,861,829 issued to Roberts et al. Roberts et al. '829 disclosed a wobble plate type compressor which has a cam rotor driving device to drive a plurality of pistons. The slant or incline angle of the slant surface of the wobble plate is avaried to change the stroke length of the pistons which changes the displacement of the compressor. Changing the incline angle of the wobble plate is effected by changing the pressure difference between the suction chamber and the crank chamber in which the driving device is located.
In such a prior art compressor, the slant angle of the slant surface is controlled by the pressure in the crank chamber. Typically this control occurs in the following manner. The crank chamber communicates with the suction chamber through an aperture and the opening and closing of the aperture is controlled by a valve mechanism. The valve mechanism generally includes a bellows element and a needle valve, and is located in the suction chamber so that the bellows element operates in accordance with changes in the suction chamber pressure.
In the above compressor, the pressure in the suction chamber is compared with a predetermined value by the valve mechanism. Referring to FIG. 3, the pressure is maintained within a small range SR1 in the suction chamber. However, when the predetermined value is below a certain critical value Pa, there is a possiblity of frost forming on the evaporator in the refrigerant circuit. Thus, the predetermined value is usually set at a pressure P1 higher than the critical value to prvent frost from forming on the evaporator. In the graphs, T.sub.1 is the time at which the thermal load is increased. Before time=T.sub.1, the thermal load is being determined.
However, since suction pressures above this critical value are higher than the prssure in the suction chamber when the compressor operates at maximum capacity, the cooling characteristics of the compressor are inferior to those of the same compressor without a variable displacement mechanism. Furthermore, since the range of the pressure change CR1 in the crank chamber becomes large, lubricating oil contained in the crank chamber may flow into the external refrigeration circuit through the suction chamber and cylinders. This reduces the heat-exchanging ratio of the evaporator.
One solution to resolve these disadvantages is disclosed in our copending patent application Ser. No. 918,065 filed on Oct. 14, 1986. This application disclosed a variable capacity mechanism for a slant plate type compressor which controls the capacity of compressor while uniformly maintaining the pressure in the crank chamber, as shown in FIG. 1. In these compressors, the capacity adjusting mechanism operates to control the capacity of the compressor using a predetermined pressure value of P2 in the suction chamber which is lower than the critical pressure value Pa. Therefore, the cooling characteristics are improved. However, the pressure range in the suction chamber SR2 increases. Therefore, the temperature changes of the air flowing out of the evaporator and into the passenger compartment of automobile also increases.
Roberts et al. '829 discloses a capacity adjusting mechanism used in a wobble plate type compressor. As is typical in this type of compressor, the wobble plate is disposed at a slant or incline angle relative to the drive axis nutates but does not rotate, and drivingly couples the pistons to the drive source. This type of capacity adjusting mechanism, using selective fluid communication between the crank chamber and the suction chamber can be used in any type of compressor which uses a slanted plate or surface in the drive mechanism. For example, U.S. Pat. No. 4,664,604 issued to Terauchi discloses this type of capacity adjusting mechanism in a swash plate type compressor. The swash plate, like the wobble plate, is disposed at a slant angle and drivingly couples the pistons to the drive source. However, while the wobble palte nutates only, the swash plate both nutates and rotates. The term slant plate type compressor will therefore be used to refer to any type of compressor, including wobble and swash plate types, which use a slanted plate or surface in the drive mechanism.